The “Running W” was invented to make horses fall during filming

Don’t worry, the Running W is now illegal. During its use, a wire was attached to a horse’s leg and when pulled taught due to running, would trip the horse and cause it to tumble!

Many horses suffered serious injuries such as broken legs and necks, so it was deemed illegal due to its inhumane uses and consequences.

Examples of the Running W can be seen in classic western films such as “Escort West” and “Outlawed Guns”.

(Source)